Phototriggered Mass Migrating Motions in Liquid Crystalline Azobenzene Polymer Films with Systematically Varied Thermal Properties

A family of homopolymers and random copolymers consisting of an azobenzene (Az)-containing methacrylate and hexyl acrylate were synthesized varying the copolymerization ratio. With increasing the content of hexyl acrylate, the temperature range of the liquid crystalline (LC) state was monotonously lowered. These polymer films were subjected to an exposure to the patterned irradiation to evaluate the mass transport behavior. This approach revealed that the efficient mass migration is strongly coupled with the thermal properties. Under the optimized conditions, two types of irradiation procedures were adopted: (i) two-step irradiation involving the UV (365 nm) light illumination and subsequent patterned visible light (488 nm of argon ion laser or 436 nm Hg line) and (ii) one-step patterned irradiation with UV light. After the irradiation with a photomak, the former method caused the mass transfer from the illuminated regions to shaded ones, whereas the latter led to the migration in the inversed direction. In both procedures, the migration consistently proceeded at the boundaries from the smectic A phase regions to the isotropic ones in the photochemically generated phase patterns. More general guidelines are proposed to exert the efficient mass transport in the LC Az polymers by adjusting the polymer design and temperature.